Before you paint, rub the whole thing down with windex or anything with ammonia in it to get rid of grease. If there is any grease on it (even the simple touch of your finger), it is likely to make it look like a bad job... When the grease reacts with the paint, it'll give an ugly orange peel texture rather than a nice smooth coat.
Orange peel:
http://www.vintagebus.com/howto/colo...nge-peel-2.jpg
If there aren't any glossy parts, then you shouldn't need to even touch sandpaper if you're really careful with the grease AND you are a patient painter.
Multiple, thin, light coats takes longer but believe me, it beats a run that you'll have to sand and redo which is a MASSIVE PITA.
Also, since the gun is going to be in your sweaty soldier hands and be put through a make-shift war, you should consider baking on your paint.
Baking your paint basically quadruples its bond to the surface, and it's exponentially more resistant to wear from rubbing.... If you know what you're doing, you can take the gun apart and paint only the externals you want to paint, and paint them individually. I suggest taking an old metal baking pan, covering it with wax paper, and resting the part(s) on it. Spray, and immediately throw it in your oven, being careful not to get anything to touch the paint and ruin the coat. You want the oven hot, around 250F, and you want them in there for about 20 seconds for every thin coat you spray.
If you decide not to take it apart for painting, and if you can fit your whole gun in the oven, it should still be ok to bake. You're only heating up the paint and the surface it's bonding to, not the entire part.
Just remember whatever you do, paint patiently, you'll pat yourself on the back for it.
-Adam
